President Donald Trump says the United States has signed what he called 'another very important part of the Immigration and Security deal with Mexico,' without revealing any details about its contents.

He said Monday on Twitter that the unspecified measure is one 'the U.S. has been asking about for many years.' He added that it would need to be approved by Mexico's congress, and threatened that if the measure fails there, he will once again return to his threat of imposing new tariffs on Mexican goods.

The latest statements follow a deal announced Friday that calls for Mexico to dispatch 6,000 troops to its border with Guatemala to halt the flow of migrants, and gives the United States new authority to force asylum seekers to stay in Mexico while their legal cases in the United States are pending.

Trump has long criticized Mexico for not doing enough to cut the number of people arriving at the southern U.S. border and ahead of Friday's announcement had been pledging to impose 5% tariffs, with future escalations, unless the Mexican government took more action.